1. Field of the Invention
The present invention relates to an electronic still camera.
2. Description of the Related Art
Electronic still cameras are classified into two types: the first type involves analog recording using a floppy disk and the second type involves digital recording using a semiconductor memory (RAM etc.). Digital recording has various advantages over analog recording. For instance, digital recording produces no image deterioration due to copying, and does not involve any rotating mechanism for image recording, which contributes to making the camera compact.
The solid state image sensors which are employed in ordinary commercial cameras typically have about 400,000 pixels in view of the cost of the solid state image sensors. The solid state image sensors having pixels of that quantity can reproduce nearly satisfactory pictures when viewed on existing monitors, but the reproduced pictures do not have a satisfactory quality when output on a HD (High Definition) monitor or hard-copied. A Hi-vision solid state image sensor (having about 200 million pixels) for high definition is expensive and is not therefore suitable for commercial electronic still cameras.
There is a still picture input system which uses an inexpensive solid state image sensor having about several hundred thousand pixels and accomplishes high resolution by shifting the solid state image sensor. According to this system, the red (R) filter of a color filter disk is located in front of the solid state image sensor and a picture signal is read from the solid state image sensor. This signal is subjected to a predetermined process in a pre-processor, and is then converted into a digital value by an A/D converter. The picture signal is then processed to have a recording format by a digital signal processor before being recorded on a recording medium. When R data of one frame is recorded by the above operation, the solid state image sensor is moved by a fine distance to obtain a high resolution, and then the recording operation similar to the above is performed to obtain a plurality of frame data for R. When processing for R is completed, the color filter disk rotates to put the green (G) filter in front of the solid state image sensor. The G signal is processed and recorded in the same manner as the R signal The same processing is likewise executed for the blue (B) filter.
A still picture with high resolution is acquired in the above manner. Since, however, the solid state image sensor used in this system is a monochromatic device and requires a rotary filter for picking up image information in a field sequential image pickup system, the camera body is large and the sensor can pick up the image of only a still object. In this respect this solid state image sensor is not suitable for an electronic still camera which picks up a moving object as a still picture.
When a black and white object such as characters is imaged by an electronic still camera using an ordinary solid state image sensor having about 400,000 pixels, color moire occurs due to the color filter array and a clear picture cannot be obtained. To reduce the color moire, a crystal optical filter is generally provided. But this method deteriorates the resolution and is undesirable.
Digital electronic still camera systems, which records a still picture as a digital signal in a card type semiconductor memory as a recording medium, have been announced or put on the market by several companies. This semiconductor memory card is detachably mounted into the camera body. The unit price for semiconductor memories per bit is still expensive and the memory capacity is still insufficient, so that an optical disk, magneto optical disk, hard disk, magnetic tape, magnetic bubble memory, etc. are used or usable to record data for a long period of time or record a vast amount of data. Imaged data can be recorded on the semiconductor memory card as well as the secondary recording medium for each still picture.
Those electronic still cameras can transfer digital picture data to the monitor of a personal computer or a work station and display thereon. The digital picture data is typically converted into an analog signal input to the RF terminal or video terminal of an ordinary monitor of the TV standards and displayed on the monitor. In this case, the reproduced images are those images which are formed on the imaging area of the image sensor at the imaging time, and this area is fixed.
The auxiliary recording medium such as a semiconductor memory card, disk, and tape can record imaging data for each still picture. One cannot see, at a glance, the contents of the digital system, such as a memory card, and the above video floppy system, unlike the conventional photographic print. To see the contents, the user should place the recording medium in a reproducing apparatus and reproduce it, or write a memo on the package.
Jpn. Pat. Appln. No. 2-234492, entitled "Digital Electronic Still Camera System" discloses as an image retrieval method, a technique of using the conditions at the imaging time to improve the efficiency of image retrieval. The imaging conditions include the date of the imaging, white balance, the amount of incident light, focus, aperture, zoom, the use or non-usage of flash, humidity, atmospheric pressure, ID of the imaging camera and the type of the lens. On the reproducing side, the imaging conditions are indicated in the form of questions to allow a retriever to answer the questions, or the retriever should input the imaging conditions himself or herself. For instance, probable questions ask if the imaging was done in a room or outside the room, and if the object is a person, scenery or printed matter. The reproducing section read the imaging condition data required by the questions performs the necessary operations and reproduces images from the one with the highest probability.
Electronic still cameras, which digitize analog image information acquired from an image sensor and then compresses the digital data before recording, have already been commercialized.
The zoom function is widely used in home video movie cameras and compact cameras. There are two zooming methods for home video movie cameras: the first method uses a zoom lens and the second one is electronic zooming that performs operations on picture signals output from the image sensor. While the zoom lens can acquire good images at the time of zooming, a larger lens is needed for higher magnification, which stands in the way of making cameras compact. The electronic zooming, on the other hand, does not require a zoom lens and can thus contribute to accomplishing light and compact video movie cameras.
In performing an electronic zooming process on an image, as the image is spatially sampled by the image sensor, interpolation between sample pixels should be performed in accordance with the magnification. In this case, a typical method is to prepare data of pixels to be interpolated through interpolation using actual pixel data acquired by the image sensor. To execute pixel interpolation in real time, a bi-linear interpolation which requires a small circuit scale is widely used.
An example of this bi-linear interpolation is described in, for example, IEEE Transactions on Consumer Electronics Vol. 37, No. 3, August 1991, "An Electronic Zoom Video Camera using Image scanner control." This zooming however impairs the band of the original signals, thus deteriorating the resolution. The existing video movie cameras output or record image information as analog signals, not as digital image data in compressed form.
With regard to image compression, compression using block coding is employed as the international standard for motion pictures and still pictures and the adaptation of this system to a Hi-vision still picture storage device, a TV telephone, TV conference system and so on is studied. In conducting electronic zooming on those stored or coded digital images on the reproduction side,-block deformation originated from the block coding would be sensed by human eyes, degrading the image quality.